This invention pertains to a fluid control valve, and more specifically to such a valve which is constructed to operate in response to the pressure difference that exists between a pair of fluid-communication ports provided in the valve. A preferred embodiment of the proposed valve is disclosed herein in conjunction with load-clamping apparatus of the type which is mountable on the carriage in a lift truck, in which setting the valve has been found to have particular utility.
There are numerous situations in which it is desired to utilize a fluid control valve whose operating condition at any given point in time is determined by the pressure difference which exists between a pair of spaced apart points in a fluid circuit. For example, it has been found to be desirable to employ such a valve in the setting of load-clamping apparatus of the type which is mountable as an attachment on the carriage in a conventional industrial lift truck. In this setting, the desirability of using such a valve stems from the desire to maximize both the speed and efficiency of such apparatus by supplying and exhausting fluid to its clamping cylinders (typically two such cylinders) in response to the actual loaded and unloaded conditions of the clamping arms in the apparatus. It will be apparent that such loaded and unloaded conditions may be determined from the difference in pressure which exists in fluid in the two main conduits used for supplying fluid to and exhausting it from such cylinders.
A further consideration, and one that applies to the setting of load-clamping apparatus, is that it is often desirable to have a control valve of the type generally indicated shift from one control condition to another control condition when one preselected pressure difference exists, and to have the reverse action take place when another, and perhaps significantly lower, preselected pressure difference exists.
A general object of the present invention is to provide a unique differential fluid control valve which takes these considerations into account in a practical and satisfactory manner.
More specifically, an object of the invention is to provide a fluid control valve which is capable of performing accurately and quickly all of the functions discussed above.
Thus, an object of the invention is to provide a control valve of the type generally outlined which will accurately respond to a preselected pressure difference between a pair of ports in the valve to shift a spool therein from one control condition to another.
Another object of the invention is to provide such a valve in which an adjustment may be made so as to change the specific amount of pressure difference which is required to cause such shifting of the spool.
A further object of the invention is to provide a valve of the type indicated wherein the spool, once shifted as just indicated, returns to its initial control condition only when the sensed pressure difference drops below another preselected pressure difference which is significantly lower than that which was required to cause initial shifting of the spool.
According to a preferred embodiment of the invention, the proposed valve includes a body in which there is mounted a spool that is shiftable between a pair of control positions. This spool includes first and second working surface areas, on its opposite sides, on which pressure fluid may act to shift it toward these control positions. A spring acting on the spool urges it normally toward one of such positions.
The pressure difference of interest which is to be utilized for shifting the spool is sensed at a pair of ports in the body, and fluid passages are provided in the body for communicating betweens these ports and the opposite sides of the spool.
A unique feature of the valve is that a part is provided therein which is interposed between the first working surface area on the spool and the passage which communicates with this working surface area -- this part being constructed to enable exposure (to pressure fluid within such passage) of only a portion of the first working surface area with the spool in its control position to which it is biased by the spring. This part is further constructed to enable exposure of the full extent of the first working surface area to such pressure fluid with the spool shifted toward its other control position. This construction promotes operation whereby the spool must be exposed initially to a relatively large sensed pressure difference before it shifts from that control position to which it is normally biased. However, once it has shifted, and because there is now a larger portion of the first working surface area which is exposed to that pressure fluid which initially shifted the spool, a significantly lower pressure difference is required to hold the spool in its shifted position.
Another unique feature of the proposed valve is the incorporation of what is referred to as changeable-condition pressure-balancing means for the spool which, with the spool in its normal or nonshifted position, exposes both the second working surface area, and that portion of its first working surface area which is not exposed by the part just mentioned, to the same pressure fluid. This construction minimizes spool resistance to movement from its normal position. Such an arrangement also maximizes the accuracy with which the spool responds to that preselected pressure difference which is intended to cause the spool to shift away from its normal position. The pressure-balancing means further performs whereby, once the spool begins to shift away from its normal position toward its other control position, it blocks communication between the opposite sides of the spool so as to allow the full sensed pressure difference to act on the spool.